Another example of what you are talking about is the Mongols’ outward expansion after Genghis Khan united the various clans in the late 12th and early 13th centuries. The Hanseatic League in the Baltic region was a federation that made war on other states in the late medieval period.
If you’re looking for a counterexample, the Holy Roman Empire is not a great one, but on the whole I’d say that the component parts of it were not really at each others’ throats during much of the Middle Ages.
